Ceramic titanate piezoelectric composition and method for preparing same



United States Patent" 2,989,482 CERAMIC TITAENATE PIEZOELECTRIC COMPO- SITION AND METHOD FOR PREPARING SAME Walter S. Miller, Elmon't, N .Y., assiguor to American Bosch Arma Corporation, 'a corporation of New York No Drawing. Filed Nov. 4, 1958, Ser. No. 771,734 16 Claims. (Cl. 252-62.9)

The invention relates to an improved method of making polycrystalline ceramic titanates which exhibit useful piezoelectric and other electrical properties.

In recent years, considerable elfort has been expended in developing certain polycrystalline ceramics of the perovskite type structure for useful piezoelectric, ferroelectric, and dielectric usage. Many of these ceramics of the type here under consideration have been tested with various addition agents designed to improve the operating characteristics of the polycrystalline material. But in every instance it has been found necessary to perrnanently polarize the ceramics by the application thereto of a rather high unidirectional DC. voltage during the manufacturing process. It has also been determined that the piezoelectric properties of the ceramics are maintained only as long as the crystal is of the tetragonal symmetry. If the temperature of the crystal is raised above the Curie point or transformation point of the crystal, the remnant piezoelectric and ferroelectric prop.- erties are destroyed and the material must be reactivated by the reapplication of a similarly strong polarizing electric field. The Curie points of these ceramics are also generally low, in the neighborhood of 120 C., thus considerably limiting their application. Finally, it has been recognized that the ceramic titanates suifer the disadvantage of losing a relatively substantial degree of their sensitivity over a period of time. For instance, barium titanate ceramic bodies may lose as much as 30% of their sensitivity over a period of approximately five to .six months.

In applicants co-pending application, entitled Plastic Titanate Piezoelectric Compositions, Serial No. 771,722, filed of even date, there is disclosed a method for chem.- ically growing a polycrystalline titanate to form a plastic mass having piezoelectric properties without recourse to the required polarization step, and which overcomes many of the above-mentioned disadvantages of the ceramics. In that application, it was disclosed that the addition of lithium salts and other additives to a titanate-polyethylene glycol slurry partially reduced the titanate in an exothermic reaction to a complex titanate crystal. The heat of reaction aided by the presence of the vlithium ion yielded a complex plastic crystal having a peculiar dipole alignment and a resultant inherent polarization. Other additives, such as selenous acid, were also utilized to provide negative or positive ions for an ionic imbalance in the final crystal. It is believed that the final titanate-titanite crystal held the various additive ions physically and chemically within the crystal structure, and that various properties were obtained depending on the proportions of additives used. However, the crystalline masses describedin the co-pending application are plastic in nature making them unsuitable for many applications requiring a ceramic body and additionally the chemical reduction ,as itoithe present compounds has been found to 'be insufficient standing alone to promote polarity.

It is thus a primary object of the invention to provide a new method of making a polycrystalline polarized ceramic body exhibiting useful piezoelectric properties, without recourse to the step of placing the material in a strong electric field to polarize it. Itis also an object of the invention to provide a vmethod of making a polycrystalline polarized ceramic body wherein ,the compo- 2,989,482 Patented June 20, 19 61 nent mix is chemically processed so as to give a polar; ized ceramic on firing the mix to its ceramic state. .It is also an object to provide a method of producing acerarnic piezoelectric body having an unusually high Curie or transformation point. Other objects of the present in vention will become apparent from consideration of the present specification and claims.

In accordance with the above mentioned objects, appl i cant has developed a method of producing a piezoelectn'c ceramic body of the perovskite type structure from a titanate-polyethylene glycol slurry, said body having an inherent dipole alignment, and thus positive or ne gw tive polarity, induced both by certain chemical reduc-y ing components controlling crystal growth and the sube sequent firing step. The reducing components preferably compounds containing the lithium ion yield a peculiar crystal structure, which though not polarized in the plastic state, is polarized on being fired in a reducing or oxidizing atmosphere. The reduction with the lithium ions below the Curie point at 140-230 F. partially grows a complex titanate crystal throughout the plastic mass, the growth of which is completed by firing the plastic mass in a reducing or oxidizing atmosphere resulting in the said polarized crystal. Whether a reducing or oxidizing atmosphere is used depends on the nature of and amount of additives used in the prior chemical reducing step. The resultant polycrystalline ceramic olarized mass is believed to be a complex titanate-tit crystal, the amounts of reducing components added and reducing or oxidizingatmospher'e being sufficient to generally only partially reduce the titanate crystal. said crystal incorporates physically and chemically the growth controlling reducing components and otherjadditives and perhaps components of the long chain poly; ethylene glycol forming the above mentioned slurry. Each reducing component, additive or agent along with the particular firing schedule utilized is influential in ,effecting a radical crystal structure exhibiting specific electrical properties. The transformation point or Curie point of these crystals is believed to be in the neighbor hood of about 500 F. Also, since the polarization pf the crystal is induced both chemically and by being fired in a reducing or oxidizing atmosphere the piezoelectric or other electrical properties specific to the crystals are not adversely affected by the application of heat or the passage of time. i

Generally, barium titanate comprises a major. propontion of the starting material ranging from about 30 {to and preferably from about 60 to 90%. Additional amount of other well-known isomorphic titanates o'f th e perovskite crystal structure may be admixed with the barium titanate as additives influencing the Curie point. These titanates include calcium, strontium titanates, and active zirconium and lead titanates. Amounts of these additives range from 0-60% and preferably are in the range of 2-l5%. Such isomorphic mixtures of barium titanates with other metallic titanates may be grouped and termed piezoelectric metallic titanates following the terminology of the Howatt Patent 2,507,253. From a practical standpoint, crystals having a predominant portion of barium titanate exhibit the most useful electrical properties, and thus most of the examplesof the present invention disclose a predominant percentage of barium titanate in the starting mixture. As a preferred conveyor and plasticizingagent.forthe titanate starting mix forming the starting slurry, applicant utilizes a liquid polyalkylene glycol such as poly.- ethylene glycol, Nos. l00-200-300-400 (Union Carbide Corporation), in adequate amounts to form said plastic slurry. mass, elastic in structure, of the consistency of SAEil SO, grease. However, thinner or ,thicker mixtures-may he Slurry as used here is preferably a stiff viscous used if it appears advisable for various reasons in the final product. Operable, but not preferred, slurry producers include water and certain acids such as HCl, and H 80 but it is believed that the polyethylene glycol or decomposition products thereof enter into chemical or physical combination in the final crystal and thus actively contn'bute to the physical and electrical properties obtained.

The reducing component for activating the above noted piezoelectric titanates is preferably selected from a compound providing lithium ions, such as the lithium halides (LiCl, LiBr), U 8, and LiOH, in the amount of from 1 to 40%, and preferably from about to 15%, of the final dry mixture, and provides the desired heat of reaction for chemically growing the titania crystal to its polarized state. In some cases, the lithium ion may be provided by lithium titanate which provides a dual function as an isomorphic titanate modifier for barium titanate, as well as. an active reducing agent. Applicant has found that among the group 1A elements, the lithium ion and particularly the lithium salts give the preferred reduction and heat of reaction necessary to yield the plastic mass which becomes polarized on firing. Secondary operable reducing agents also include zinc chloride, specifically desirable for yielding an ionic interchange type crystal useful for an alkaline battery. It should be noted that LiF and other lithium salts have been proposed before as fluxes in the crystallization of BaTiO In the present invention, the lithium ion actively enters the polycrystalline titania structure, chemically and physically reacting with the titanate, and is not a mere flux. In preparing the plastic compositions of the present invention the lithium compound or reducing agent is added last to the mixture to provide a heat of reduction which can be utilized by the entire mixture.

The exothermic heat of reduction or heat of reaction produced by the addition of the reducing component is generally in the range of l40-230 F. and is a function of the amount of reducing component utilized. The heat of reduction is generally well below the transformation point of the reduced titanite and/ or titanate.

In certain of the compositions, to achieve specific properties, other additive such as CuCl, CuO, ZnO, and selenous acid are utilized to provide a surplus of predominantly positive or negative crystalline structures. Forinstance, selenous acid provides negative ions for an imbalance in the total crystal to provide rectifier action and/or photo-sensitivity.

It will be noted in the following examples that if the compositions are totaled with respect to the number of electrons in the outer rings of each component that there will be a surplus or a deficit in the total. For instance, a predominance of positive or negative ions in the crystal structure will influence the dipole alignment achieved in the partially reduced titanate crystal, and thus alter the various work functions of the final polycrystalline mass.

The specific percentages of reducing components and additives in particular used in the examples of the invention are critical, both with respect to the method utilized to yield a polarized crystal and the particular properties obtained. It is believed that in some instances, the percentages may be varied as much as one-fifth, but generally, the percentages recited are critical. The end products obtained when the percentages are varied more than one-fifth are very unstable and tend to degrade quickly.

For instance, in the aforementioned co-pending application Serial No. 771,722 entitled, Plastic Titanate Piezoelectric Compositions, Example 2 recites adding 5% LiCl to a slurry comprising 90% BaTiO and 5% LiOH, giving a heat of reduction of about 165 F. and yielding a polarized plastic mass. In the present application, Example 2 recites adding 6% LiCl to a slurry comprising 86% BaTiO;,, 3% PbTiO and 5% LiOH, giving a heat of reduction of F., and yielding a non-polarized plastic mass which becomes activated on being fired. The properties displayed by the two crystals are also different.

Following the chemical reduction of the titanate slurry, the mixture is subjected to a firing schedule in an oxidizing or reducing atmosphere to produce the final ceramic product. The temperature range during firing is from 2500 to 3000 F. Whether an oxidizing or reducing atmosphere is used depends on the particular nature of and amounts of reducing components, and additives used during the chemical reduction process, and the properties desired. It is the combination of a particular and critical chemically influenced reduction plus continued reduction or oxidation during firing which produces specific and particular properties.

In accordance with the patent to Roup 2,520,376, a layerized BaTiO ceramic body was produced by alternate firing in anoxidizing and reducing atmosphere, with a rare earth oxide as a reduction promoter. In distinction, in applicants process, use is first made of the exothermic chemical reduction with lithium ions followed by firing in either a reducing (hydrogen) or oxidizing (oxygen) atmosphere. Applicants compounds are thus partially reduced throughout the plastic mass, the crystal growth being subsequently completed to a polarized state by the said firing schedule.

In general, the present plastic compositions are pro duced from the several components by the following method. The unfired BaTiO or piezoelectric titanate mixture is thoroughly mixed or blended to a homogeneous mass or powder and sufiicient polyethylene glycol is slowly added to form a stiff viscous slurry. Subsequently, the reducing component preferably in the form of a lithium salt is added, which causes an exothermic reaction generating a heat of reduction influential in controlling the crystal growth. The reducing component'where LiCl is used is believed to partially reduce the titanate, for example to form barium trihydrochloride titanite. The level of temperature elevation achieved is primarily dependent upon the amount of lithium ion introduced, but may secondarily be influenced by the introduction of other additives, such as copper halides and oxides, zinc oxide and selenous acid. These latter additives are generally introduced in the original mix with the titanate compound.

After the heat of reaction has subsided, the moist crystalline plastic mass which exhibits unusual moisture retention characteristics, is ready as discussed above for firing in a hydrogen or oxygen atmosphere. The firing temperatures range around 2700 F. and the atmosphere is as mentioned designed to promote oxidation or reduction, depending on the component materials used. The ceramic mass can be fired in any required shape, thus yielding the final desired product.

The crystal ceramic mass performs many difierent functions depending on the component additives, titanatcs and reducing agents used, and the particular firing schedule and atmospheres. Among these functions are the generation of a voltage with the application of force or pressure, or by the application of heat, the voltage generated being a function of the applied stimulus, and the development of voltage and current storage devices, photosensitive elements, non-linear resistors, rectifiers, amplifiers, capacitors and modulators, dielectrics, ferroelectric ceramics, diodes, semi-conductors, N- and P-type, and thermocouples.

Example No. 1

Percent Lead titanate-PbTiO 60 Barium titanate-BaTiO 30 Lithium chloride-LiCl 10 All materials with the exception of LiCl are thoroughly mixed. The glycol is added. LiCl is added. Heat of reduction is in the area of 200 F, with the material being Percent Barium titanateBaTiO 86 Lead titanate-PbTiO 3 Lithium hydroxideLiOH Lithium chlorideLiCl 6 This material with the exception of LiCl was mixed with polyethylene glycol added to form a plastic slurry. LiCl was then added with a heat of reduction of 180 F. When fired in a hydrogen atmosphere, this material became an eflicient non-linear resistor with voltage sensitive characteristics, a thermistor, and at the application of heat will change its resistance, or alternately, increase in current will change its resistance. It exhibits utility of being used as a solid state relay. It also exhibits utility as an N type material or alternately a P type material by changing the concentration of lead titanate to the extent that it could be used for transistor ordiode application.

.The {firing temperature was about 3000 F.

Example N a. 3

. Percent Barium titanate-BaTiO 60 Lead titanate-PbTiO 5 Lithium hydroxideLiOH 30 Lithium chlorideLiC1 5 This mixture with the exception of LiCl was combined as a homogeneous mass with adequate polyethylene glycol added to make a plastic slurry. The heat of reduction at this point was approximately 140 F. The color was slightly on the tan side. The LiCl was then added and thoroughly mixed with an increasing heat of reduction to be noted of approximately 180 to 200 F. This mixture, when fired in a hydrogen atmosphere of 3000 F. for a period of two hours yields an N type semi-conductor mixture which has very fast response time with respect to heat. This mixture has as its end use thermocouple application and active useas a thermostat. Additional utility has been found by using this composition as a rectifier capable of operating at approximately 2000 F.

Example No. 4

Percent Barium titanate-'BaTiO 80 Calcium titanate-CaTiO 2 Lithium titanateLiTiO 3 Lithium hydroxideLiOH 5 Lithium chloride-LiCl All materials with the exception of LiCl'were thoroughly mixed and polyethylene glycol 200 was added to make a st-iflf plastic paste. reduction was in the neighborhood of 180 F. and remained at that figure for approximately five minutes. This material when fired in oxidizing atmosphere showed dense crystal structure and will generate a voltage upon LiCl was then added and the heat of the application of pressure without resorting to polarization electrostatically during cooling by raising the material above its Curie point. It has found application in transducers, pressure sensitive devices, vibration sensitive devices, and as a storage me di a.

The firi g temper ur r g wa tram .2500 to 3000" F.

of the polypropylene glycol. With glycol the heat of reduction was 200 F. The mixture was fired in an oxidizing atmosphere and created a moderately high value of dielectric constant. It was extremely stable ternperature-wise. It showed a typical hysteresis of ferroelectric devices, was slightly pressure sensitive and exhibited diode action.

The firing temperature range was from 2500 F. to 3000 ,F.

Example No. 6

Percent Barium titanate--BaTiO '70 Lithium hydroxideLiOH 10 Calcium titanate--CaTiO 2 Zirconium titanateZrTiO 5 Zinc chlorideZnCl 3 Selenous acid-H SeO 10 These elements, with the exception of the ZnCl and the H SeO were thoroughly mixed to provide a homogeneous mass with polyethylene glycol being added ,after thoroughly mixing. The heat of reduction at this :point was approximately F. Then the ZnCl-and H SeO were added successively. The heat of reduction with the two elements, ZnCl and H SeO added rose above 200 F. This material generated a gas which was volatile and should not be exposed to openflame. Pieces of this material which have been fired in a reducing atmosphere of hydrogen at 2700 F. have demonstrated utility as solid state thermionic generators in that theappl-ication of heat with a temperature gradient of from 32 F. to 2200 F. will produce a voltage not in excess of $4 of -a volt DC. with a current of 150 micro amperes maximum dependent on the degree of heat applied. This material was polarity sensitive to heat in that a cylindrical rod of arbitrary dimensions1 inch by A inch, for examplewill exhibit a positive or negative polarity dependent upon the end the head is applied. This composition was definitely a N type semi-conductor material which when joined with a given type material as a junction area shown highly eflicient diode action and in one instance indicated highly efficient transistor action with a current gain of the sample piece being in th neighborhood of 100. i

This mixture when compounded with the exception of the LiCl and selenous acid created a violent bubbling action which will settle down with the addition of the selenous acid and then the .LiCl. There are .three successive stages of heat reduction which are respectively at 135 F., 180 F. and 203 F. The end mixture has as its use when fired in an oxygen atmosphere a very high K dielectric material which exhibits capacities in the order of 10,000 mrn f. per square centimeter-by 5 mils. thick. Its temperature co-eificient is fairly reasonable being estimated in the area of 20 parts per millionpcr 10 F. It is possessed with the typical ferro electric 7 hysteresis and exhibits under certain frequencies a rectangular wave shape on the oscilloscope.

The firing temperature was 27 F.

I claim:

1. The method of polarizing and promoting crystal growth of a perovskite type titanate ceramic comprising (a) reacting a mixture consisting essentially of at least 60% of an unfired piezoelectric metallic titanate selected from the group consisting of alkaline earth titanates, zirconium titanate, lead titanate, and combinations thereof, and fi-om 140% of an exothermic inorganic chemical component containing lithium ions selected from the group consisting of lithium halides, LiOH, Li S and LigTlOa homogeneously mixed with sufiicient liquid polyalkylene glycol to form a thick plastic slurry, and (b) sub sequently firing the partially reduced reaction product at a temperature in the range of about 2500 F. to 3000 F.

2. The method according to claim 1 wherein said firing is conducted in a reducing atmosphere.

3. The method of claim 1 wherein said firing is conducted in an oxidizing atmosphere.

4. The method according to claim 1 wherein said metallic titanate is predominantly BaTiO and includes at least one additional titanate modifier selected from the group consisting of Li TiO PbTiO CaTiO SrTiO and ZrTiO 5. The method according to claim 1 wherein the liquid polyalkylene glycol is polyethylene glycol.

6. The method according to claim 1 wherein the component containing lithium ions is present in amount of about 5-15% based on the dry weight of the reduced reaction product.

7. The method according to claim 1 wherein in addition to lithium ions the exothermic inorganic component contains zinc ions.

8. The method of promoting crystal growth and polarizing a piezoelectric ceramic titanate comprising (a) forming a thick plastic homogeneous slurry of suflicient liquid polyalkylene glycol and unfired metallic piezoelectric titanate selected from the group consisting of alkaline earth titanates, zirconium titanate, lead titanate, and combinations thereof, and adding to said slurry at least one ionic imbalance promoter selected from the group consisting of H SeO ZnO, TiO CuCl and CuO; (b) reacting in amount of from 1.01 to 67% of the amount of metallic titanate of an inorganic component selected from the group consisting of lithium halides, LiOH, U 5 and Li TiO with said slurry, and (c) subsequently firing the reaction product at a temperature in the range of about 2500 to 3000 F.

9. The method as claimed in claim 8 wherein said imbalance promoter is added last to said slurry.

' 10. A polarized ceramic crystal consisting essentially of the reaction product of lithium free metallic titanates of the perovskite type structure and which contain substantial amounts of barium titanate and LiCl in the amount of about 10% by weight of the dry mixture, the reaction being conducted in the presence of sufiicient liquid polyethylene glycol to form a thick plastic slurry, and said reaction product being subsequently fired in an oxygen atmosphere at 2700 F.

11. A polarized ceramic crystal consisting essentially of in approximately the following percentages by weight of the dry mix, the reaction product of Percent BaTiO 86 PbTiO 3 LiOH 5 homogeneously mixed with sufiicient liquid polyalkylene glycol to form a thick plastic slurry, and

Percent Percent BaTiO 60 PbTlOg 5 LiOH 30 homogeneously mixed with sutficient liquid polyalkylene glycol to form a thick plastic slurry. and

. Percent LiCl 5 said reaction product being fired in a hydrogen atmosphere at about 3000" F.

13. A polarized ceramic crystal consisting essentially of in approximately the following percentages by weight of the dry mix, the reaction product of Percent BaTiO CaTiO 2 LiTiO 3 LiOH 5 homogeneously mixed with sufiicient liquid polyalkylene glycol to form a thick plastic slurry, and

Percent said reaction product being fired in an oxygen atmosphere at about 2700 F.

14. A polarized ceramic crystal consisting essentially of in approximately the following percentages by weight of the dry mix, the reaction product of Percent BaTiO 80 CaTiO; 3 Zl'TiO3 4 PbTiO 3 TiO 3 homogeneously mixed with suflicient liquid polyalkylene glycol to form a thick plastic slurry, and

Percent said reaction product being fired in an oxygen atmosphere at about 2700 F. v 15. A polarized ceramic crystal consisting essentially of in approximately the following percentages by weight of the dry mix, the reaction product of Percent BaTiO a. !70 CaTiO3 2 ZrTiO 5 LiOH 10 with ZnCl 3 and H SeO 10 in the presence of sufficient liquid polyethylene glycol to form a thick plastic slurry, said ZnCl and H SeO being added in that order and the reaction product being fired in a hydrogen atmosphere at about 2700 F.

16. A polarized ceramic crystal consisting essentially of in approximately the. following percentages by weight References Cited in the file of this patent of the dry mix, the reactlon product of Percent UNITED STATES PATENTS Ba'I iO 75 2,520,376 Roupe et a1. Aug. 29, 1950 SrT O3 4 5 2,598,707 Matthias June 3, 1952 3 1 2,602,753 Woodcock et a1. July 8, 1952 LIOH 15 2,750,657 Herbert et a1. June 19, 1956 I with 2,803,519 Karan Aug. 20, 1957 2 3 2 1 OTHER REFERENCES and Synthetic Organic Chemicals, Carbide and Carbon LiCl 3 Chem. Cor-p., N.Y., 12th ed., 1945, pp. 19-23.

in the presence of sufficient liquid polyethylene glycol to B P Y Muljddomain form a thick plastic slurry, said H23603 and Licl being 15 Barium-Titanate Single Crystal, Physlcal Revlew, vol.

added in that order, the reaction product being fired 108, 2, October 1957, PP- in an oxygen atmosphere at about 2700 F. 

8. THE METHOD OF PROMOTING CRYSTAL GROWTH AND POLARIZING A PIEZOELECTRIC CERAMIC TITANATE COMPRISING (A) FORMING A THICK PLASTIC HOMOGENEOUS SLURRY OF SUFFICIENT LIQUID POLYALKYLENE GLYCOL AND UNFIRED METALLIC PIEZOELECTRIC TITANATE SELECTED FROM THE GROUP CONSISTING OF ALKALINE EARTH TITANATES, ZIRCONIUM TITANATE, LEAD TITANATE, AND COMBINATIONS THEREOF, AND ADDING TO SAID SLURRY AT LEAST ONE IONIC IMBALANCE PROMOTER SELECTED FROM THE GROUP CONSISTING OF H2SEO3, ZNO, TIO2, CUCL AND CUO, (B) REACTING IN AMOUNT OF FROM 1.01 TO 67% OF THE AMOUNT OF METALLIC TITANATE OF AN INORGANIC COMPONENT SELECTED FROM THE GROUP CONSISTING OF LITHIUM HALIDES, LIOH, LI2S AND LI2TIO3 WITH SAID SLURRY, AND (C) SUBSEQUENTLY FIRING THE REACTION PRODUCT AT A TEMPERATURE IN THE RANGE OF ABOUT 2500* TO 3000* F. 